Novel aminofunctional dimer technology improves the performance of state-of-the-art polyamide hot melt adhesivesby: Angela Smits, Hans Ridderikhoff, Wolfgang Geuking
For years dimerized fatty acids have been used in polyamides, polyesters, epoxy resins and amine compounds. They are obtained by the conversion of unsaturated fatty acids (from natural oils and fats) through a combination of pressure, temperature and catalysis, followed by purification processes. Starting with the C18 grade acids that nature usually provides, dimer acid is a molecule with 36 carbon atoms, making it by far the longest dioic acid available. The benefits that the hydrocarbon nature and non-crystallinity of these C36 diacids bring to coatings and adhesives have to do mainly with the low level of Tg (vitreous transition) and its hydrophobic character. This implies flexibility, hydrolytic resistance, resistance to thermo-oxidation, moisture repellency and adhesion to a wide range of substrates.
Derivatives such as dimeric fatty diols and polyester polyols, obtained from dimerized fatty acids, are used, for example, in different polyurethane applications, to improve durability, flexibility and hydrophobicity.
New functional amino biologically based building blocks have been developed (Figure 1), expanding the range of hydroxyl and functional acid fatty dimers and derived polyesters. This new technology brings benefits for many applications. In polyamide hot melt adhesives, the use of dimer diamine allows greater freedom in the formulation, which in turn makes a wider adjustment of the melting point possible. This increases hydrophobicity, causing greater moisture repellency and greater adhesion to plastics. The new functional amino materials expand the application possibilities and, in addition, provide environmental benefits due to their renewable nature (100% renewable carbon), their low volatility and durability.
Dimer fat diamine
Polyamide adhesives are used in a wide variety of applications, such as footwear, repair and sealing of electrical appliances, textiles, assembly and packaging of furniture. The amine component provides intermolecular bonding and cohesion resistance, a high melting point, green resistance and adhesion to polar substrates. Dimeric fatty acids are used to give flexibility and pressure absorption and achieve better flow and moisturization, as well as better adhesion to low-energy substrates. Other dioecious acids of the carboxylic acid component are used to increase the melting point.
The availability of dimer diamine allows the freedom to use dimer in the amine component, which gives more options in terms of the diacids used, thus providing greater control of the melting point. In addition, the use of both dimer acid and dimer diamine in a single formulation allows extremely low glass transition temperatures without compromising oxidative stability, even better moisture resistance, greater adhesion to low-energy substrates and greater flexibility, characteristics that are useful in applications related to electrical appliances, packaging and textiles in very abrasive environments.
Thermal and hardness properties
A first comparison was made between the use of dimer acid or dimer diamine in a formulation. The polyamide hot melt adhesives (listed in Table 1) were synthesized by heating the acid component and then adding the amine component for the reaction. When the possible boiling stage passed, the temperature was gradually increased to 230°C (or more when the viscosity so requires). Reactions were continued until an acid value of 9 mg KOH/g was reached, representing a molecular weight of 12500 g/mol. The heat fusion adhesives were analyzed with the differential scanning calorimetry technique, to determine the glass transition (Tg) and melting temperatures. Shore-D hardness was also measured.
As expected, the presence of the dimeric structure C36 greatly reduces both the melting point and the glass transition temperature, as seen in the second line of Table 1, making polyamide more flexible. The replacement of dimer acid by dimer diamine (and, consequently, hexamethylene diamine with adipic acid) increases the melting point and hardness, as shown in the third line of Table 1, while maintaining a low Tg (vitreous transition). However, this can be explained by the fact that there is a difference in the total dimeric content (83 vs. 78%), caused by adding an excess of carboxylic acid.
In another experiment, the results of which are shown in the fourth line of Table 1, the total dimer acid content was reduced by incorporating adipic acid to a level of 78%, as used in the dimer diamine formulation. This material shows a hardness and tg similar to that of the dimer diamine-based material, as shown in the third line. However, dimer acid-based polyamide no longer has a well-defined melting point but a melting path. This causes undesirable softening and skewering of the adhesive. Thus, the dimer diamine extends the freedom in the formulation, allowing a greater adjustment of the melting point without compromising performance. By altering the acid component in the formulation, the melting point can be further adjusted.A second comparison of dimer diamine with several amines was made. Hexamethylene diamine (HMDA) was used as the reference diamine. Polyeteramine MW400 was used as a flexible diamine and aminoethyl piperazine as a secondary amine for better adhesion to plastics. A small percentage of EDA had to be used in all formulations to obtain a solid polymer. The polyamide hot melt adhesives shown in Table 2 were prepared, the reactions were continued until reaching an amine value of 7 mg KOH/g, representing a molecular weight of 14000 g/mol.
The dimer diamine reduces hardness and Tg while maintaining a high melting point, similar to the soft block of polyetheramine, thus considerably expanding the temperature variations of the application.
Moisture resistance
Moisture absorption has been analyzed by allowing adhesive samples to absorb moisture in water at 25°C for a week, and by analyzing weight gain. The mixture of dimer acid and dimer diamine absorbs less water (Figure 2). As expected, polyetheramine has a higher affinity for moisture, which softens the polymer under wet conditions, reducing cohesion resistance.
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Adhesion to plastics
Adhesion properties were analyzed by measuring adhesion strength by resistance to breakage over a range of substrates 500 μm thick. Adhesion to beach wood did not cause breakage (at 3.2 MPa) in the four samples. The results shown in Figure 3 show that compared to HMDAs, the other three amines show an improvement in ABS-like adhesion. On nylon the dimer diamine implies a greater improvement, which causes a failure of the substrate. It is curious that the hydrophobicity provided by dimer diamine improves adhesion to the substrate of lower polarity: untreated polypropylene.
Conclusion
The technology of functional amino dimerized fatty acids can be used well in polyamide adhesives, thus expanding their application possibilities. These new bio-based building blocks provide several ecological and performance advantages due to their renewable nature, lower volatility and good thermal oxidation stability for more durable applications. In polyamide hot melt adhesives, the use of dimer diamine allows greater freedom in the formulation, which makes possible a greater adjustment of the melting point. Moisture repellency and adhesion to plastics are better. These characteristics, added to its great flexibility, make the dimer diamine an ideal component to improve state-of-the-art applications such as encapsulation or electronic sealing and packaging, footwear, textiles and automotive applications.
* Croda, Netherlands. For more information, please contact [email protected]
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